1. Technical Field
The present invention relates in general to data communication and in particular to data communication in a computer network. Still more particularly, the present invention relates to method and system in a computer network for maintaining source-route information at a router located within a default communication path between two devices when the router is bypassed by shortcut communication between the two devices.
2. Description of the Related Art
Generally speaking, two broad categories of communication protocols are utilized in data communication networks: broadcast protocols and unicast (point-to-point) protocols. In data communication networks that implement a conventional broadcast protocol, a source station transfers data to a destination station by broadcasting a data packet containing the destination station""s address to all stations attached to the network. In response to sensing a data packet containing its address, the destination station receives the data packet from the network; all other stations ignore the data packet. Because every data transfer between a source station and a destination station in a broadcast network requires broadcasting data packets in this manner, broadcast networks are designed to support only a relatively small numbers of stations. A conventional broadcast network""s limited bandwidth would be overwhelmed by the number of broadcast packets if a large number of stations were attached.
Because of the bandwidth limitations of individual broadcast networks, broadcast networks are often implemented as a series of interconnected subnetworks (or subnets). Such a network architecture is also a natural outgrowth of the desirability of interconnecting each new local area network (LAN) with preexisting LANs. Communication between subnetworks that each form a portion of a larger internetwork is accomplished by devices called routers, which provide layer-3 (i.e., network) connectivity between subnetworks according to the International Organization for Standardization Open Systems Interconnection (ISO/OSI) model. Thus, when a source station desires to transfer data to a destination station in a different subnetwork of a broadcast internetwork, the source station broadcasts, on its subnetwork, a data packet containing the address of a destination station. The broadcast data packet is received by each station in the source station""s subnetwork, including a router. In response to sensing the destination station""s address, the router receives the data packet from the source station""s network and broadcasts the data packet on the destination station""s subnetwork, to which the router is also attached. As described above, in response to sensing a data packet specifying its address, the destination station receives the data packet from its network, while all other stations on the destination station""s subnetwork ignore the data packet.
Because of the size limitation and inherent inefficiency of networks that implement broadcast protocols, there has been increased interest in unicast protocols that permit point-to-point data communication. One emerging network technology that permits such connection-oriented communication is asynchronous transfer mode (ATM). An ATM protocol is defined in xe2x80x9cATM User-Network Interface (UNI) Signaling Specification, Version 4.0,xe2x80x9d which is promulgated by the ATM Forum Technical Committee and incorporated herein by reference. In an ATM network, a source station negotiates a connected path to a desired destination station before the source station proceeds to transmit its data to the destination station. The ATM protocol defines the communication required to establish such a point-to-point connection. Once a point-to-point connection has been established, the source station transmits its data only to the destination station using unicast frames. Thus, unicast protocols, such as ATM, address the shortcomings of conventional broadcast protocols by providing high speed, low bandwidth communication.
When only communication speed and data throughput are considered, it would be desirable to replace the installed base of non-ATM protocol networks (e.g., Wide Area Networks (WANs), Local Area Networks (LANs), Internet Protocol Networks) with high speed connection-oriented network technologies such as ATM. However, because the expense of replacing the enormous number of installed broadcast networks is prohibitive and because some of the non-ATM protocol networks have features that are not directly supported by ATM protocol networks, industry has taken a gradual approach to upgrading network technologies. Under this approach, the communication industry has opted to move toward ATM protocol networks while simultaneously continuing to support the vast installed base of non-ATM protocol networks and the network and link layer protocols operating on the non-ATM networks until the non-ATM networks are phased out. The key to this strategy is empowering the ATM protocol networks to be able to support non-ATM protocols, and to be able to support non-ATM features that users (and user""s systems) have come to expect and rely upon.
The telecommunications industry has opted to provide such support and supply such features via various overlay schemes. These overlay schemes, while representing a number of different implementations, all support any non-ATM capability with a logically separate protocol that is logically overlaid onto a base ATM protocol network. The logically overlaid protocol is then utilized to allow non-ATM protocol networks to interact with ATM protocol networks as if the ATM protocol networks were a part of, and hence recognize the protocols and support the features of, non-ATM protocol networks.
One scheme for supporting conventional LAN technology in the context of ATM includes the implementation of a Logical IP Subnetwork (LIS), which is a ISO/OSI layer-3 subnetwork within an ATM network. The ATM network serves as a direct replacement for the physical LAN segments connecting traditional layer-3 source and destination stations (collectively, end stations) and routers. Communication between end stations within the same LIS is accomplished utilizing the ATM protocol. In order for a source station to establish a connection with a destination station within the LIS, the source station must learn the ATM address of the destination station. Accordingly, the source station sends an ATM Address Resolution Protocol request (ATMARP xe2x80x94Request) to an ATM Address Resolution Protocol (ARP) Server, which resolves the destination station""s layer-3 address (i.e., the address by which the destination station is identified with respect to its subnet) into an ATM address that the source station can utilize to obtain a point-to-point connection. By contrast, communication between end stations in different LISs is performed as if the end stations resided in different traditional LANs. The source station transmits the data traffic to a router based on the layer-3 address of the destination station. After determining the appropriate destination address (and protocol) by reference to a table, the router then forwards the data to the appropriate destination LIS or to a second router, if the router on the source LIS is not a member of the destination LIS.
Another mechanism for supporting conventional broadcast technology in an ATM environment is an emulated LAN (ELAN), which supports the use of IEEE 802 LAN protocols, such as Ethernet and Token Ring protocols, within an ATM protocol network. Communication within an ELAN is governed by a LAN emulation (LANE) protocol, such as xe2x80x9cLAN Emulation Over ATM Version 2xe2x80x94LUNI Specificationxe2x80x9d (AF-LANE-0084.000), which is promulgated by the ATM Forum and incorporated herein by reference. The LANE protocol defines a service interface for higher layer (that is, network layer) protocols, which is identical to that of legacy LANs, and requires that data sent across the ATM network are encapsulated in the appropriate LAN MAC (medium access control) packet format. LANE does not emulate the actual media access control protocol of the specific LAN concerned (that is, CSMA/CD for Ethernet or token passing for Token Ring LANs). In other words, a LANE protocol makes an ATM protocol network appear and behave like an Ethernet or Token Ring LAN. The rationale for layering protocols in this manner is that no modifications to higher layer protocols are required in order to enable their operation over an ATM network and no changes are required in network layer drivers since the LANE service presents the same service interface as existing MAC protocols.
The LANE protocol referenced above defines the operation of a single emulated LAN (ELAN), which may be one of multiple ELANs on a single ATM network. A conventional ELAN is composed of a LAN Emulation Server (LES), LAN Emulation Clients (LECs) of the LES, and a Broadcast and Unknown Server (BUS) and is managed by a LAN Emulation Configuration Server (LECS). Often, LECs serve an intermediary between a legacy LAN and an ATM network in which an ELAN has been established, communicating with the attached legacy LAN utilizing conventional LAN protocols and communicating with other entities on the ELAN as described below.
To establish communication between two LECs on a ELAN, the source LEC first tries obtain the ATM address of the destination LEC, given the destination LEC""s MAC address. To obtain the ATM address of the destination LEC, the source LEC issues a LANE Address Resolution Protocol Request (LE_ARP_Request) to the LES. Provided the destination station has previously registered its MAC address/ATM address pair with the LES serving the ELAN, the LES returns the ATM address of the destination LEC to the requesting LEC in an ELAN Address Resolution Protocol Reply (LE_ARP_Reply). The source LEC can then use the received ATM address to establish a connection to unicast data to the destination LEC, a so-called data-direct Virtual Channel Connection (VCC), and transmit its data to the destination LEC via the VCC.
If the destination LEC has not registered with the LES, the source LEC communicates with the destination LEC using a conventional LAN methodology mediated through a Broadcast and Unknown Server (BUS). The LEC first sends its data packets to the BUS, which broadcasts the data packets to each station in the ELAN. As in a legacy LAN, the broadcast data packets are received from the network by the destination LEC and are ignored by other stations on the network. Exactly the same process is used if a destination station for data communication is on a legacy LAN or an ELAN in a different layer-3 subnetwork. In these cases, the broadcast data packets are received by a router connected to the ELAN and forwarded via layer-3 protocols to the destination, as described hereinabove, just as if the ELAN were a legacy LAN.
In prior art networks, communication between a LIS attached station and an ELAN attached station of an ATM network also proceeds by router-connected paths. Without a mechanism for resolving the ATM address of a destination station, a source station attached to a LIS cannot exploit the ATM network to which it is attached, and data packets are sent to a router connecting the layer-3 subnets, which forwards the data packets via layer-3 protocols to the layer-3 subnet in which the destination station resides. The routed path through the network may involve forwarding the data from the source station to the destination station through one or more routers, where each such forwarding event is referred to as a xe2x80x9chop.xe2x80x9d Transferring data from the source station to the destination station in this way, via a series of hops, subverts the benefits of the ATM because the source station is not exploiting the ATM connection-oriented infrastructure.
In order to eliminate hops in data communication within a LANE environment, the Next Hop Resolution Protocol (NHRP) was developed. NHRP forms Annex C of the Multi-protocol over ATM (MPOA) specification entitled xe2x80x9cMulti-protocol Over ATM, Version 1.0.,xe2x80x9d (AF-MPOA-0087.000), which is promulgated by the ATM Forum and incorporated herein by reference in its entirety. NHRP is a protocol that allows an entity, such as a host, connected to a layer-3 subnet of an ATM network to determine the ATM address of the next hop associated with a destination station. The next hop may be, for example, the destination station itself or a router xe2x80x9cnearestxe2x80x9d the destination station that provides egress from the ATM network to an LAN attached destination host. Utilizing the next hop ATM address, NHRP permits an ATM VCC shortcut path for data packets to be established between stations in lieu of the default routed path.
In ATM protocol networks, it is frequently the case that LAN devices communicating with one another are behind LANE edge devices (i.e., bridge devices located between one or more LAN interfaces and one or more LECs). If only LANE and NHRP are implemented, the LANE edge devices must be full function routers in order to establish an ATM VCC in accordance with the LANE and NHRP protocols. To eliminate this requirement, MPOA was developed. MPOA allows LANE edge devices to perform internetwork layer forwarding and establish ATM VCCs without requiring that the LANE edge devices be full function routers. As described in detail in the MPOA specification incorporated by reference above, MPOA defines MPOA clients (MPCs), which issue queries for shortcut ATM addresses, and MPOA servers (MPSs), which service such shortcut queries in order to identify the optimal exit point for data packets from the ATM xe2x80x9ccloud.xe2x80x9d
When a router in an MPOA protocol network first receives a data packet from a source station (e.g., a Token Ring attached device), the router examines the data packet and caches the source-route information contained in the data packet in association with the source station. However, if the router receives no data packets from the source station over a long period of time (typically 2 to 5 minutes), the router will age out the source-route information from its cache. Thus, in many conventional MPOA protocol networks, a router cannot learn or maintain a source-route to an end station without receiving data packets from that end station.
However, as discussed above, an MPOA protocol network permits an MPC to establish an ATM VCC bypassing the source station""s default router utilizing an ATM address supplied by an MPS. The present invention recognizes that bypassing a source station""s default router in this manner can cause two problems. First, in the absence of a known source-route, data packets received by the default router that target the source station must be broadcast as explorer packets on the ELAN in which the MPC resides, thereby severely degrading network performance. Second, an MPOA shortcut (i.e., ATM VCC) cannot be established or maintained in the reverse direction (i.e., with the source station as the target) because the source-route information must be present at the default router in order to discover the MPC associated with the target station utilizing the discovery mechanism of the MPOA standard.
Some layer-3 protocols have mechanisms that can be employed to address this problem. For example, the IP_ARP function can be used for IP (Internet Protocol). An MPS can perform an IP_ARP to obtain the source-route to an IP end station because the IP end station will respond to the MPS with a packet that contains the source-route. However, not all layer-3 protocols have such a mechanism. For example, IPX (Internet Protocol exchange) does not include such a mechanism. For IPX, an MPS cannot send a packet over the ELAN that would generate a response to the MPS from the end station for which the source-route is needed. Therefore, the present invention solves the problem for such protocols (e.g., IPX) in an MPOA network.
To overcome the aforementioned and other shortcomings of the prior art, the present invention provides an improved communication methodology in a data communication internetwork including a source station, an end station, and a default router for the source station. In this data communication internetwork, a default communication path including the default router is defined between the source station and the destination station. During operation of the data communication internetwork, a shortcut data communication path bypassing the default router is established between the source station and the destination station, such that data packets transmitted by the source station are not received by the default router. After the shortcut communication path has been established, a control frame containing source-route information regarding the source station is transmitted to the router, and the source-route information is stored at the router. In this manner, the default router can efficiently direct data packets to the source station and facilitate the establishment of shortcut communication paths terminating at the source station even though the default router is bypassed by a shortcut communication path.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.